#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define all(a) (a).begin(), (a).end()
#define sz(a) (int) a.size()
#define pb push_back
#define ff first
#define sc second
#define pii pair<int, int>
#define f(i, l, r) for (int i = (l); i < (r); i++)
#define double ld
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
string s; cin >> s;
vector<int> posj, posi, pref(n + 1);
for (int i = 0; i < n; i++) {
pref[i + 1] = pref[i];
if (s[i] == 'J') posj.push_back(i);
else if (s[i] == 'I') posi.push_back(i);
else {
pref[i + 1]++;
}
}
int ans = 1e9;
int ind = -1;
for (int i = 0; i < n; i++) {
if (s[i] == 'J' && ind+k<sz(posj)) {
ind++;
int L = -1, R = sz(posi);
while (R - L > 1) {
int M = (L + R) / 2;
if (M < (k - 1) || posi[M - k + 1] < posj[ind+k-1]) {
L = M;
continue;
}
int cntt = pref[posi[M - k + 1]] - pref[posj[ind+k-1]];
if (cntt>=k)R=M;
else L=M;
}
if (R < sz(posi)) ans = min(ans, n - 3 * k - i - (n - posi[R] - 1));
}
}
cout << (ans == 1e9 ? -1 : ans) << '\n';
return 0;
}
